1. Field of the Invention
The present invention relates in general to a suspended substrate quadrature coupler for combining two receive or transmit amplifiers for increased power output, reduced port VSWR and improved reliability.
2. Description of the Background Art
There is a need for low cost, low insertion loss, high-power handling capability quadrature couplers that can be inserted into an integrated circuit module assembly (IMA). For receiver modules, insertion loss is critical to minimize the noise figure contribution between the antenna and the first low noise amplifier. Any coupler insertion loss will directly add to the overall noise figure of the receiver module. For transmitter modules, low-loss and high power handling is critical if the coupler follows the power amplifier. For transmit applications, the coupler may need to handle over one hundred watts of RF output signal from two power amplifiers. The coupler needs to be low loss so that maximum power is transferred from the power amplifier to the transmit antenna and to minimize the wasted transmit power heating the coupler.
Prior art microwave couplers include microstrip Lange couplers, waveguide couplers and stripline couplers. Low insertion loss Lange couplers require precision lithography to maintain close spacing between coupler fingers. These tight spacing requirements are difficult to achieve in low cost printed circuit processes. In addition, Lange couplers require thick substrates for low loss and to increase the line-to-line spacing needed for a 3 dB quadrature coupler. Lange couplers are also not suited for low cost printed circuit implementation because of the need for crossovers to interconnect the alternate fingers. Waveguide couplers can handle high power RF signals with low loss but are very large at cell phone frequencies and are not suitable for integration with active devices.
Stripline couplers are the most common approach for quadrature 3 dB couplers at cell phone frequencies because of the well matched even and odd mode coupled transmission line impedance. These couplers require multilayer printed circuit fabrication techniques. The dielectric loss of the circuit board material is critical for low loss stripline couplers because all of the electric field energy is stored in the dielectric material. Suspended stripline couplers can be fabricated as a single layer printed circuit board but normally have the disadvantage of very different even and odd mode velocity in coupled transmission line structures. The electric field energy is stored mostly in air for the even mode while there is more electric field energy stored in the dielectric material for the odd mode, thus slowing the odd mode velocity relative to the even mode. The difference in coupled line phase velocity causes generally poor performance for simple suspended stripline couplers. Thus, there is a need for an improved suspended stripline coupler that provides closer mode velocity matching and improved coupler performance.